In recent years, in an information recording/storing apparatus such as a hard disk unit, the magnetic recording density has been made higher. There has increased a request for a highly sensitive and high S/N ratio magnetoresistive device in which a high rate of change of resistance can be obtained in a low magnetic field. The “highly sensitive” means that a resistance change (Ω) per unit magnetic field (Oe) is large. As the magnetoresistive device has a larger magnetoresistive (MR) change and is more excellent in soft magnetic characteristic, the magnetoresistive device becomes more sensitive. In addition, in order to obtain a high S/N ratio, it is important to make the thermal noise as low as possible, and to improve the frequency response. It is therefore undesired that the device resistance becomes large. When the magnetoresistive device is used for a hard disk unit, it is preferable that the device resistance is a value of about 5 Ω to 30 Ω.
A magnetoresistive film such as a spin valve film has been known as the magnetoresistive film. The spin valve film has a configuration in which a ferromagnetic layer, a non-magnetic layer, a ferromagnetic layer and an antiferromagnetic layer are laminated in this order. In one of the ferromagnetic layers, magnetization is pinned by exchange bias or the like using the antiferromagnetic layer in advance. The other ferromagnetic layer is formed so that the magnetization can be rotated easily by an external magnetic field (signal magnetic field or the like). When only the magnetization of the other ferromagnetic layer is rotated by an external magnetic field, the relative angle between the magnetization directions of the two ferromagnetic layers is changed so that a large magnetoresistive effect can be obtained. This is described in the document “Appl. Phys. Vol. 69, 4774 (1991)”.
A ferromagnetic layer whose magnetization rotates easily in accordance with an external magnetic field is generally called “magnetization free layer”, “magnetic field sensitive layer”, “magnetization free layer”, or the like. On the other hand, a ferromagnetic layer whose magnetization is fixed is called “pin layer”, “magnetization pinned layer”, or the like. A non-magnetic layer is called “spacer layer”, “interface adjustment intermediate layer”, “intermediate layer”, or the like.
A CPP (Current-Perpendicular to Plane) system in which a current is applied to a magnetoresistive film perpendicularly is expected to have a higher MR change rate than a CIP (Current-In-Plane) system in which a current is applied into a film surface of a magnetoresistive device. Particularly in order to attain information playback at a high recording density over 100 Gbit/inch2, the device size is about 0.1 micron square. In this event, in the CIP system, it is difficult to secure a satisfactory S/N ratio due to a problem such as increase in resistance or heating. In order to overcome this, a value over 30% is required as the MR change rate.
A tunneling magnetoresistive (TMR) film has been known as a magnetoresistive film having an MR change rate over 20%. In the TMR film, a tunneling film is used as the intermediate layer of the spin valve film, and a current is applied perpendicularly to the TMR film surface. However, due to the resistance of the TMR film reaching a value of about 100 Ω or more, frequency response property required for reading a signal cannot be obtained. In addition, shot noise increases so that the S/N ratio cannot be improved.
According to the CCP spin valve, the resistance can be suppressed to be low because the intermediate layer is of metal. In addition, all the current passes through the interface between the ferromagnetic layer and the intermediate layer. Accordingly, the MR effect can be enhanced. However, in this structure, the resistance is too low to secure a required output voltage. In addition, due to a high ratio of parasitic resistance, MR observed in the device is several percentages in spite of the large MR effect. In order to solve these problems, there has been proposed a current-confined CCP spin valve using an insulating layer (current confinement layer) in which a metal conduction path is formed (see JP-A-2002-208744). When an insulating layer in which a metal conduction path is formed in or near an intermediate layer where a magnetoresistive effect appears, the resistance of a region contributing to MR can be made larger than the parasitic resistance. As a result, the MR value in the device can be increased, while the resistance can be adjusted to improve an output V (V=MR×“resistance R”×“sense current I”).
In the current-confined-path CPP spin valve, a microscopic metal conduction path (metal path) is used. The microscopic metal path is apt to be affected by electro-migration (EM), and the resistance is apt to increase locally so as to heat locally and lead to elementary thermal diffusion. Therefore, it is a problem to obtain long-term reliability.